1. Field of the Invention
The present invention relates to a method and system for ultrasound excitation of structures so as to reduce friction for ease of movement. More specifically, the present invention relates to the field of materials handling where performance can be increased considerably by ultrasound excitation of screening materials, the reduction of friction between bulk goods and their respective containers, or to a reduction of the mechanical resistance between materials which can thus reduce the wear and the energy input, respectively, in mechanical motion processes.
2. Description of the Related Art
The industry has a number of uses for which it is desirable to reduce the friction between the particles and/or between the particles and a system, which is in contact therewith. Some examples for such uses are:                The ultrasound screening, where the performance can be increased considerably by ultrasound excitation of the screening material. The performance in response to the ultrasound screening is a function of the clogging tendency of the screening materials. The material openings are kept free by means of the use of ultrasound because the static friction is transferred into the smaller dynamic friction due to the ultrasound motion and because powder bridges are broken.        The transport of bulk goods and colored powder in tubes or on platforms. The friction between the bulk goods and the platform or the conduit reduced by the ultrasound excitation is reduced. The volume flow rate can thus be metered better and the performance can be increased.        The excitation of interfaces between moving particles or between fixed and moved surfaces. Generally, the transition from the static friction to the dynamic friction caused by the use of ultrasound leads to a reduction of the mechanical resistance and can thus reduce the wear and the energy input, respectively, in mechanical motion processes.        
According to the state of the art, it was normal until now to adapt the natural vibration frequency of the mechanical body, which is to be made to vibrate, to the converter frequency for the purpose of ultrasound excitation. Such a screening system can be found in DE 4418175, for example.
However, when using this approach, it is problematic that the tuning of the mechanical body, which is to be made to vibrate, to the converter frequency is difficult and is connected with much effort. Already common manufacturing tolerances, in particular at welding or other connecting points, or fluctuations of the acoustic parameters, such as e-module, speed of sound and density, lead to mechanical bodies with slightly different natural frequencies, which already differ from one another to the extent that the operation of a plurality of screens, for example, is impossible with an ultrasound converter according to the state of the art.
Upon transitioning to more complex mechanical bodies, the individual resonances thereof are no longer clearly developed, for the most part, and one attains a mountain of resonances, as is shown below. This vibration behavior generally does not oppose a performance-boosting ultrasound excitation. It is known from EP 0 567 551 B1, for example, to excite the frames of screening systems to a vibration outside of the resonance frequency.
If, nonetheless, problems oftentimes arise in response to the operation of ultrasound-excited systems, which are not tuned to the frequency of the ultrasound converter, this is the result of the presently used ultrasound generator technology, where the phase angle is used for controlling the generator.
This control principle works better, and the more clearly the zero crossing can be determined in response to the change of sign of the phase, that is, in particular with high-quality resonance systems, which, in turn, can be achieved only with exactly tuned resonators without high attenuation effect.
Vice versa, resonances, which do not identify a clear zero crossing of the phase, are not recognized and the control fails. If the quality or the phase information deteriorates during the operation, the phase control can fail completely and the generator goes into overload.
While a use of the phase control is indeed advantageous in very high-quality systems, as must be used in response to ultrasound welding, e.g., this approach becomes susceptible and instable when the quality of the vibrating system is not sufficient. Accordingly, it must thus be ensured that this is the case by means of extensive individual adaptation of the vibrating system to the desired resonance frequency.
A further problem in response to a resonance excitation is that the resulting resonance amplitude is determined so as not being capable of being controlled, particularly in complex resonance systems. This is problematic because this variable determines the power loss, which in turn leads to the heating of the system. An uncontrolled heating as such is already disadvantageous in many cases, because a sintering of the powder, or of the bulk goods, is boosted. This problem increases in response to materials which already become soft or start to melt at low temperatures.
Furthermore, the quality of the excited system is a function of the temperature. It is thus possible, in response to resonance excitation for the heating of the system, to improve the quality, which, in turn, leads to a higher resonance amplitude and thus to a further heating, which further improves the quality.
Based on this state of affairs, the problem of providing a method and a system for ultrasound excitation arises, with which the excitation of arbitrarily complex structures and, in particular, also of a plurality of screens, are made possible in response to the lowest possible heating.
Accordingly, there is a need for an improved system and method for ultrasound excitation comprising the features of a connection between a generator, an ultrasound converter, and at least one mechanical system to be excited, and passing through a frequency range for determining an operating point. At each approached frequency, the power consumption of the system to be excited determines a current, and/or a voltage emitted by the generator, which is measured using a sensor such that a measurement value of the sensor renders the power output to the system to be excited, and performing an ultrasound excitation at the determined operating point or its immediate surroundings. The device further comprises a memory for storing target values for the power supplied to the total system input, and of parameter values for voltage, current, and frequency, in which the desired target values or ranges are achieved.